The invention can be used to test and tune resonant electric structures such as antenna or transponders, particularly transponders for “Radio Frequency Identification” systems, shortly RFID systems.
Automatic and wireless identification systems such as transponder systems are more and more important and gradually replace well-known barcodes. Such a transponder system comprises a reader and at least one transponder. A transponder comprises firstly a chip comprising e.g. all relevant data identifying a product and secondly a transponder antenna capable to operate in the relevant frequency band. The transponder antenna is part of a resonant electric structure realized by a resonant electric circuit. In this respect ISM bands such as the American UHF band ranging from 902 MHz to 928 MHz can be used. A reader comprises a reader antenna. By means of the transponder antenna and the reader antenna a bi-directional and wireless communication between the transponder and the reader is enabled.
An exchange of information with a transponder being a few meters away from the reader needs processing means operating in the radio frequency range, particularly above 800 MHz. These systems are as already mentioned commonly called “Radio Frequency Identification” systems, shortly RFID systems.
In order to obtain desired information from a passive transponder the reader initiates the reader antenna to emit electromagnetic waves having a wavelength λ wherein said emitted electromagnetic waves cause an electromagnetic field. The resonant electric structure of the transponder is designed to have a resonance frequency corresponding to the wavelength λ and thus absorbs energy from the electromagnetic field. The energy absorbed is used to operate the chip. Switching a load-modulation switch on and off results in an amplitude modulation of the electromagnetic waves generated and emitted by the reader and transmitted to the transponder. If this switching procedure of the load-modulation switch being controlled by data stored in the transponder, these data will be transmitted to the reader. This way of data transmission is called load modulation.
In the patent document DE 101 17 249 A1 a wireless test device for a coil being disclosed that coil being provided for usage in a transponder. The coil is subjected to a radio frequency (RF) field of varying frequency. The RF field is influenced and modified by the coil resulting in a modified RF field and the modified RF field being detected by an antenna. As the coil is located very close to the antenna the coil is positioned in its near field.
In the patent document U.S. Pat. No. 6,104,291 a method to test RFID tags being disclosed. The RFID tag is positioned in the near field of a reader antenna to communicate with the reader. Various commands and functions of the RFID tag are tested and information such as programming, identification, and the like are transmitted to the tag.